[cfe-commits] PATCH: Enhance array bounds checking

[Kaelyn Uhrain]

Hi Ted,

I've attached a new version of my patch that moves the pointer arithmetic
portion of the -Warray-bounds improvements into a separate flag,
-Warray-bounds-pointer-arithmetic. Since the remaining noise in the pointer
arithmetic check is more a case of undefined behavior that does the right
thing with most (all?) compilers than a case of being true false positives,
the plan is to have -Warray-bounds-pointer-arithmetic initially be disabled
by default and in later patches to enhance the warning by e.g. providing a
fixit note to add parens to exprs like "ptr + sizeof("foo") - 1" so they
become "ptr + (sizeof("foo")) - 1".

Cheers,
Kaelyn

On Thu, Jul 21, 2011 at 1:18 PM, Ted Kremenek <kremenek at apple.com> wrote:

> Hi Kaelyn,
>
> While I think there will be some contention here, I think this is still too
> high of a false positive rate for this warning to be on by default in the
> compiler.  Users simply aren't going to tolerate it.  I think it is
> reasonable for a codebase to adopt a strict policy for this warning, but I
> don't think a 17% false positive rate (or possibly higher) is acceptable for
> a default warning for all users of Clang.
>
> Unless you believe you think there are additional heuristics to drop the
> false positive rate down below 5-10%, I think I'm fine with proceeding as
> putting this in as an opt-in warning, and then refine from there.
>
> On Jul 20, 2011, at 11:39 AM, Kaelyn Uhrain wrote:
>
> I've attached an updated version of my patch that better handles cases
> where pointer arithmetic is done after casting a constant-size array to a
> pointer for a smaller base type (e.g. casting an int array to char*). Of the
> pointer arithmetic warnings, about 24% could be considered false positives;
> however, the actual number of false positives is quite small and 2/3 of them
> stem from the use of a single macro--if you count those as a single warning
> & false positive, the rate drops to 17%. Of the false positives most are
> from semi-questionable pointer arithmetic where a constant greater than the
> length of the array/pointer is being added to the pointer and some int > 1
> being subtracted from it, e.g.:
>
> void foo(int n) {
>   char x[5];
>   if (n > 0) bar(x + 6 - n);
> }
>
> Cheers,
> Kaelyn
>
> On Tue, Jul 19, 2011 at 11:44 AM, Kaelyn Uhrain <rikka at google.com> wrote:
>
>> I'm still looking at the pointer arithmetic warnings to determine which
>> are false positives, but I realized I screwed up my previous stats a bit as
>> I forgot to account for duplicated/repeated warnings (e.g. the same header
>> included in multiple compilation units and so generating the same warnings
>> multiple times). For unique warnings, the stats are:
>>
>> - 16.5% increase in warnings from before my patch (originally reported a
>> 24% increase)
>> - 47% of those new warnings being about pointer arithmetic
>> - 6.7% of all of the warnings emitted with my patch applied are concerning
>> pointer arithmetic. (originally reported 8.3%)
>> - The new pointer arithmetic warnings represent a 7.7% increase in
>> warnings from before my patch, not 10%.
>>
>> I'll send another email once I have a feel for how noisy the pointer
>> arithmetic warnings are.
>>
>> Cheers,
>>  Kaelyn
>>
>>
>> On Mon, Jul 18, 2011 at 5:54 PM, Kaelyn Uhrain <rikka at google.com> wrote:
>>
>>> Ted,
>>>
>>> You're welcome. I'll try to figure out what fraction of the pointer
>>> arithmetic warnings are false positives (requires a bit of manual digging on
>>> my part to determine if the code is indeed buggy or if it is valid /
>>> intended). For the overall 24% increase in warnings, keep in mind that over
>>> half of that is the existing bounds checking now being applied to cases
>>> where it wasn't before, i.e.:
>>>
>>> char *foo[5];
>>> foo[77];  // -Warray-bounds already found
>>> &foo[77];  // -Warray-bounds currently misses
>>> *foo[77];  // -Warray-bounds currently misses
>>>
>>> The function that did the bounds checking would never catch the last two
>>> cases because it would see a UnaryOperator (in the above cases for the '&'
>>> and the '*') and skip the expression instead of looking inside the
>>> UnaryOperator expression for the array subscripting.
>>>
>>> The new pointer arithmetic bounds checking only represents a 10% increase
>>> in warnings--and IMHO that is the only part where the number of false
>>> positives introduced might be an issue.
>>>
>>> Cheers,
>>> Kaelyn
>>>
>>>
>>> On Mon, Jul 18, 2011 at 5:36 PM, Ted Kremenek <kremenek at apple.com>wrote:
>>>
>>>> Hi Kaelyn,
>>>>
>>>> Thanks for the statistics.  What would be good to know is what fraction
>>>> of these are false positives (i.e., are these all real bugs)?  A small
>>>> random sample might be helpful.  A 24% increase in warnings is fairly
>>>> substantial, and we don't want to do that unless there is a real benefit.
>>>>
>>>> Ted
>>>>
>>>> On Jul 18, 2011, at 5:18 PM, Kaelyn Uhrain wrote:
>>>>
>>>> Ted,
>>>>
>>>> On Thu, Jul 14, 2011 at 5:04 PM, Ted Kremenek <kremenek at apple.com>wrote:
>>>>
>>>>>
>>>>> The only other issue: should this be controlled under a separate
>>>>> warning flag, at least initially so we can experiment with this new warning
>>>>> and see how noisy it is?  E.g. "-Warray-bounds-pointer-arithmetic".
>>>>>
>>>>
>>>> I've tested the -Warray-bounds changes against the Google codebase and
>>>> my patch increases the number of warnings from -Warray-bounds by 24%. Of the
>>>> new warnings, 57.33% are for array indexes that most likely weren't picked
>>>> up before because of unary operators like & or * (11.1% of all the warnings
>>>> now emitted), and the remaining 42.67% are from out-of-bounds pointer
>>>> arithmetic (8.3% of all the warnings from -Warray-bounds).
>>>>
>>>> Cheers,
>>>> Kaelyn
>>>>
>>>>
>>>>
>>>
>>
> <array-bounds-enhancement3.diff>
>
>
>
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